Signal receiving system



arch 17, 1936; GLBEERS 2,034,647

SIGNAL RECEIVING SYSTEM Filed Dec. 6, 19250 2 Sheets-Sheet l JQ Y AMPLIFIER INVEN OR. fl.. eers,

ATTORNEY.

17,1936. G. 1 BEERS 2,034,564?

i SIGNAL RECEIVING SYSTEM Filed Dec. e, 'lo 2 sheets-sheet 2 IL ll ATToRNEY.

Patented Mar. 17, 1936 UNITED STATES PATENT OFFICE SIGNAL RECEIVING SYSTEM Delaware Application December 6, 1930, Seriai No. 500,468

13 Claims. (CI. 25o-20) My invention relates to signal receiving systems, and it has particular relation to systems adapted to the selective reception of radio signals.

It isa well-known fact that radio signals, during their travel between a transmitting station and a receiving station, are subject, at times, to certain inuences that cause attenuation of the carrier-wave without appreciably affecting the amplitude of the side-bands and that, conversely, one or more side-bands may, preferentially, be attenuated. The phenomenon is generally known as selective fading.

Selective fading, particularly fading of the carrier-wave, causes distortion of the received signal. The distortion is of the same type as that occasioned by over-modulating the carrier at the transmitting station and it is, also, equivalent to that introduced through the use of detectors of certain types.

An object of my invention, therefore, is to provide a signal-receiving system whereby the effects of selective fading shall be minimized.

A related object is to provide a radio-receiving system such that the signal-output therefrom, at audio-frequency, shall be free from distortion of the type introduced by certain conventional demodulating devices.

An additional object of my invention is to so design a radio-receiving system that it shall function satisfactorily independently of the percentmodulation of a received signal.

The problem of adjacent channel selectivity is also demanding solution. Radio receivers are now so sensitive that signals from many far distant broadcast transmitters, the assigned frequencies of which, by reason of their geographical separation, are less than ten kilocycles apart, cannot satisfactorily be separated.

Another object of my invention, accordingly, is to provide a radio-receiver that shall have extreme selectivity.

An analogous object is to provide a radio receiving system which shall be substantially unaffected by carrier-waves other than that from a desired transmitting station.

'Ihe foregoing objects, and other objects appurtenant thereto, I prefer to accomplish by, first, providing means for deriving, from an incoming signal, a component representing the side-bands thereof and another component representing the carrier-wave. I, further, provide means for individually amplifying the several components to compensate the effects of selective fading and under or over-modulation, together with means for, thereafter, combining the amplied components to produce an audible signal that is free from distortion. For the purpose of combining the components, I prefer to make use of linear detectors since the audio output therefrom is substantially independent of the amplitude of the carrier-wave of an incoming signal unless the said amplitude falls below a pre-determined amplitude.

The improved adjacent channel selectivity of a receiving-system constructed according to my invention results from very careful filtering of the component representing the carrier-wave before combining it with the component representing the side-bands. Specifically, l' make use of one or more piezo-electric crystals for filtering purposes and I provide means whereby the crystal and the electrode structure associated therewith are rendered substantially opaque to frequencies other than that of the carrier-Wave component.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawings, in which:

Fig. l is a diagrammatic view of a simplified receiving-system comprising an embodiment of my invention, and

Fig. 2 is a diagrammatic view of a complete radio receiving-system of the superheterodyne type wherein is incorporated a preferred embodiment of my invention.

As exemplified by Fig. 1 of the drawings, an incoming radio signal, received upon an antenna I, is simultaneously impressed upon the input circuits of a radio-frequency amplifier 3, and a balanced detector stage constituted by a plurality of thermionic devices 5 and T, disposed in pushpull relation.

The input circuit of the balanced detector includes an inductor 9 and a tuning condenser Il, the grids of the detector tubes being connected, respectively, to the opposite ends of the inductor. A connection i3 extends between an intermediate point on the input inductor and a conductor l5 common to the cathodes of the detector tubes, a coupling coli l1 and a, biasing battery I9 being included therein. 'I'he potential of the biasing battery is such as to bring the detector tubes substantially to the cut-off point.

Both of the detector tubes are supplied with plate potential from a common source 2l, the

positive terminal of which is connected to an intermediate point on the primary winding 23 of an output transformer 25. A common source 21 supplies filament potential for the two tubes.

The detectors 5 and 1 are so chosen as to have substantially identical characteristics. The audio output currents therefrom, therefore, are practically equal and, since their inductive effect upon the secondary winding of the outputtransformer is balanced, no audio output is obtained by reason of the unmodified action of the detector stage alone. There is, of coursefa slight phase displacement between the audio currents in the two halves of the primary winding 23 but its effect is negligible.

To obtain an output at audio-frequencythe condition of balance must, accordingly, be so disturbed that the currentfiowing vin one half of the primary winding consistently exceeds that flowing in the other half or the phase-angle betweenthe two currents must materially be increased.

I propose, therefore, to utilize the carrier-wave per se, after appropriate amplification, for the purpose of introducing the requisitev unbalance. The radio-frequency amplifier 3 is, accordingly, provided with an output circuit, including a coil 29 that is disposed in inductive relation to the coupling coil I1, whereby the carrier-wave may be impressed upon the grids of the detector tubes, simultaneously, in phase.

In order that only the carrier-wave shall be impressed, from the amplifier 3, upon the coupling coil I1, the output circuit of the amplifier includes filtering means, such as piezo-electric crystals 3|, 33 and 35 and a switching device 31, whereby any selected one of the crystals, corresponding to the carrier-wave of a desired trans mitting station, may be used. Y

It should be clear, from an inspection of the drawings, that the detector tubes 5 and 1 are unresponsive to the carrier-wave alone, as impressed thereon from the amplifier 3, or to potentials conveyed directly thereto from the antenna, insofar as audio output is concerned. When, however, the carrier plus the side-bands and the carrier, deprived substantially of side-band frequencies, are simultaneously impressed upon the detector stage, the balanced inductive effect upon the secondary winding of the output transformer 25 is disturbed and audio potentials are developed across the secondary winding.

It should be noted, at this point, that coupling coils 9 and l1 may be interchanged without affecting the operation of the system. That is to say, the incoming signal may, alternatively, be impressed on the coil I1 while the output from the RF amplier may be impressed on the coils 9.

Since the carrier-wave itself, or locally generated oscillations at the carrier-frequency, must be combined with the modulated carrier in the balanced detector to give an audible signal, the selectivity of the system is measured by the ability of the ltering system to prevent signals at frequencies other than the carrier-wave from passing.

Inasmuch as the capacity existing between the crystal-supporting electrodes affords a path for all frequencies except zero, means must be provided for nullifying such capacity and for preventing all potentials, other than those set up across the crystalv itself by reason of its excitation by the amplified carrier-wave, from being impressed, from the coil 29, upon thebalanced detector stage. Ihe specific manner in which I prefer to accomplish the desired result, as well as the theory involved, will, hereinafter, be explained in detail.

Inasmuch as it is not commercially practicable to provide a sufliciently large number of piezoelectric crystals to correspond to all of the broadcasting stations within the range of a modern radio receiver, I find it advantageous to make use of the superheterodyne principle of reception and to apply my invention to a single intermediate frequency produced through the inter-action of oscillations from a local variable source and any incoming modulated carrier-wave.

Irrespective of any controversy as to the reality of side-bands, I have established, to my own satisfaction, at least, that the intermediate-frequency carrier-wave, through the use of my improved crystal-'filten may be substantially freed from variations representing the modulation, and, thereafter, may be combined with the unfiltered modulated intermediate frequency carrier wave to produce an audible signal. For expediency, therefore, I shall hereinafter refer to the intermediate frequency carrier wave plus the modulation simply as side-bands and to the ltered intermediate frequency carrier wave as the intermediate carrier-wave.

A receiver of the superheterodyne type, modified in accordance with a preferred embodiment of my invention, is illustrated in Fig. 2 of the drawings. The receiver includes a radio-frequency amplifier 39, a rst detector 4l, and a local source of oscillations 43.

The radio frequency amplifier, the oscillator and the first detector may be constructed in any well-known manner, since the details thereof constitute no part of the present invention.

Instead of the usual second detector, I employ a plurality of thermionic tubes 55 and 41, disposed in balanced relation, as hereinbefore described in connection with Fig. l of the drawings, having a common output circuit constituted by the primary winding i9 of a push-pull audio transformer 5|.

The detector tubes have a common tunable input circuit, including an inductor 53 and a condenser 55, and a connection including a coupling coil 51 and a grid biasing battery 59, extends between an intermediate point on the inductor and the cathode'circuit of the tubes. By proper choice of the grid bias and the grid swing, the detectors 5 and 41 may be adjusted for linear detection, Two element tubes may be used, instead, if desired.

A side-band amplifier Si is provided, the output circuit 53 of which is disposed in inductive relation to the coupling coil 51 and the input terminals of which receive excitation from the output circuit 55 of the first detector.

. In order that the intermediate carrier-wave shall be effectively freed from the side-bands, before it is impressed on the balanced detector, I interpose a filter, substantially opaque to the sidebands, between the output circuit of the first detector and the said balanced detector. Furthermore, the side-band amplifier may also be provided with filtering means, (not shown) of any desired type, whereby the intermediate carrierwave is prevented from being impressed upon the coupling coil 51.

A preferred lter is constituted by a plurality of thermionic devices 51 and 59, each having a cathode 1l, a grid 13 and a plate 15. The first thermionic device is provided with an input circuit constituted by an inductor 11 and a tuning condenser 19, the said inductor being coupled to the output circuit 65 of the rst detector.

The plate of the first thermionic device is connected to the grid of the second thermionic device through a piezo-electric crystal 8| that is disposed between a plurality of electrodes 83 and 85.

The dimensions of the crystal are so chosen that its natural frequency is the same as that of the intermediate carrier-wave and it, accordingly, is substantially unresponsive to the side-bands accompanying the said wave, and to undesired carrier-waves.

In the operation of the system as thus far described, potentials representing a plurality of frequencies appear across the piezo-electric crystal and a coupling whereby all of these potentials may be impressed upon the second thermionic device is provided by means of the capacity between the crystal supporting electrodes 83 and 85. It being, of course, understood that I desire that the intermediate carrier frequency only shall be impressed upon the second thermionic tube 69, through the piezo-electric action of the crystal, it is clear that means must be provided whereby all frequencies shall be prevented from traversing the capacitative path offered by the piezo-electric crystal holder. To that end, a circuit tuned to the intermediate carrier-wave, constituted by an inductor 8'! and a condenser 8S, and a neutralizing condenser 9, l, are serially connected around the electrodes in question and plate potential for the rst thermionic device is supplied through an intermediate point on the inductor 8l.

By reason of the tuned circuit, frequencies other than the intermediate carrier-wave, to a greater or less extent, are by-passed. The capacity between the electrodes 83 and 85, however, affords a coupling between the plate of the rst lter-tube 6l and the second tube 69. I have,

accordingly, provided the neutralizing condenser .lll through which potentials 180 out of phase with those applied directly across the crystal are simultaneously impressed upon the grid of the second tube.

The neutralization of the crystal-holder capacity is an important feature of my invention. It improves the selectivity of the system to a very substantial degree.

Like any radio receiving system, the system shown in the drawings is adapted to be controlled by automatic volume control means and it has been found that the use of an automatic volume control system is particularly advantageous in the event that the signals from any given station are greatly subject to fading since, by use, the relative amplitudes of the intermediate carrier-wave, and the side-bands, may be maintained within fairly definite limits in the receiving apparatus irrespective of changes in the carrier-wave during transmission from the distant broadcasting station.

The operation of the receiving system illustrated in Fig. 2 is essentially the same as that of the simplified system explained in connection with Fig. 1. An audible signal results only from the simultaneous presence, in the balanced detector, of the side-bands and the intermediate carrierwave, and interference from extraneous frequencies is substantially prevented.

Should the carrier fade through selective fading, the automatic volume-control (not shown) associated with the carrier-amplifier and filter could well serve to keep the amplitude of the carrier impressed upon the balanced detector thoroughly constant with the result that the signal strength would not be impaired.

Also, the piezo-electric crystal has a definite tendency toward free oscillation after the excitation provided by an incoming signal has been removed. This tendency serves to keep the intermediate carrier frequency input to the balanced detector system relatively constant in amplitude through fading periods.

It is, accordingly, apparent, that with my improved system the incoming signal may fade to a very appreciable extent since the presence of the automatic volume-control, a linear detector, and the piezo-electric crystal conjointly exert such an influence that the audio output from the system remains substantially constant. A

Since, in my improved system I am enabled to definitely fix and `control the relative percentage of carrier and side-bands that are simultaneously impressed upon a demodulating device, I am enabled to simulate the reception of signals of slight percentage modulation or of great percentage modulation, as may be desire-d. There appears, however, no present reason whereby the side-bands should be over-emphasized to simulate high percentage modulation of an incoming signal and I find it most expedient to so adjust the several amplitudes of the sidebands and the intermediate carrier wave that the system responds, as a whole, as though it were receiving signals of low percentage modulation.

Furthermore, in the event that signals from a distant broadcasting station are over-modulated, a radio receiving system such as I have herein described is very valuable. By its use the amplitude of the carrier-wave may, locally, be increased before being impressed upon the balanced detector and the effect of a deficient carrier in the received signals is compensated.

Although I have illustrated and described certain specific embodiments of my invention, many modifications thereof will be apparent to those skilled in the art to which it pertains. For example, certain of the desirable features of my invention may be attained by depriving one of the detectors 5 1 of filament potential and utilizing only the energized detector. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A signal receiving system including a pair of signal transmission circuits, means for separating an incoming modulated carrier wave into carrier component and side-band components and for applying same selectively to said signal transmission circuits, means including an automatic volume control device for maintaining substantially constant the amplitude of said side band components, means including a piezo-electric crystal and an amplifier device in that one of said circuits which transmits the carrier component for filtering and greatly amplifying' the carrier component with respect to said side-band components, a pair of line-ar detectors each having a contro-l grid and a cathode, a coupling device connected between said grids, a second coupling device constituting a common connection from said grids to said cathodes, and means including said coupling devices for electrically connecting said signal transmission circuits to said linear detectors.

2. In a signal receiving system, a plurality of detector devices each having a control grid and a cathode, a coupling device connected between said grids, a second coupling device constituting a common connection from said grids to said cathodes, a common output circuit for said devices, means for amplifying a modulated carrier wave, means for separating said carrier Wave into a carrier component and side band components corresponding to a modulation of said modulated carrier wave, means for supplying gain to the carrier component and to the side band components, the gain supplied to the carrier component being sufficiently greater in proportion to that supplied to the side band components that over-modulation and consequent distortion is substantially avoided, means including a piezoelectric device for impressing said carrier component upon said grid electrodes through one of said coupling devices, means for neutralizing the capacitive coupling effects of said piezo electric device in respect to frequencies other than that of the carrier Wave, and means for impressing said side band components upon said grids through the other of said coupling devices.

3. The method of operating a plurality of balanced linear detectors having a common input circuit for the reception of radio' signals subject to selective fading, which comprises separately and disproportionately amplifying the carrierwave component and the side-band components of the signals, the greater amplication being applied to the carrier Wave component, and thereafter impressing the carrier Wave component upon the said detectors in opposed phase relation and the other of said components upon the said detectors in like phase relation.

4. The method set forth in claim 3 wherein the carrier-Wave component is amplied to a greater degree than the side-band components to thereby offset the effects of selective fading and to' avoid the effects of over-modulation.

5. The method set forth in claim 3 characterized in that the amplification of the sideband components is held substantially constant through periods of fading.

6. The method set forth in claim 3 characterized in that the amplification of the carrier- Wave component is held substantially constant through periods of fading.

'7. In a signal receiving system, automatic volume control means to which an incoming signal is subjected, means for separating said signal into a plurality of components one of which is a carrier Wave, means for greatly increasing the amplitude of the carrier Wave component with respect to the other components, linear detector means comprising a pair of electron discharge devices, each of which is provided with an input circuit, means including a frequency selective de- Vice for impressing said carrier Wave component simultaneously and in phase opposition upon the two said input circuits, means for impressing another of said components simultaneously and in phase agreement upon the two said input circuits, and means responsive only to the joint action of the several components for producing an audible signal.

8. A device in accordance with claim 7 in which said frequency selective device includes a piezo electric crystal having a 10W decrement of o'scillation during intervals of fading.

9. A device in accordance With claim 7 in which means are provided in combination with said frequency selective device for neutralizing the inherent capacitive coupling effects thereof.

10. The method of minimizing the effects of selective fading upon the output of a receiving set of the type employing linear detector devices Which consists in regulating the volume of the incoming signal, separating said signal into carrier component and side-band components, ltering the carrier component to substantially block off all frequencies other than that of said carrier component, amplifying the carrier component more than the side-band components, impressing said amplied carrier component upon said linear detector devices at only substantially constant amplitude and in opposed phase relation, and impressing upon said detector devices said amplified side-band components in like-phase relation, Wherby to produce an audible signal.

l1. The method of overcoming the effects of fading signals upon the output of a receiving set which comprises compensating for variations in amplitude of the received signals so as to derive an output independent of non-selective fading conditions, and applying further compensation for variations in amplitude of the carrier component with respect to a side-band component to minimize the effects of selective fading conditions.

12. The method of overcoming the effects of indiscriminate fading and selective fading upon the output of a receiving set of the type employing linear detector devices which consists in regulating the volume of the incoming signal to substantially eliminate indiscriminate fading effects, separating said signal into carrier component and side-band components, amplifying the carrier component more than the side-band components, recombining said components in such manner as to derive signalling energy having a loW percentage of modulation, and impressing said recombined components upon said linear detector devices to control the operation thereof.

13. In a signaling system having an input and an output, means eifective upon the reception of signals subject to indiscriminate fading and selective fading to automatically control the amplitude of output of said system, said means comprising means for separating said signals into carrier component and side-band components, means for amplifying the carrier component much more than the side-band components, means including a piezo electric device having a low decrement of oscillation under fading conditions for ltering said carrier component, a linear detector device having an input circuit, and means for impressing the several components upon said detector input circuit, said detector device constituting means for demodulating the signals.

GEORGE L. BEERS. 

